In image-scanning devices that are used in copiers, scanners, facsimiles, and the like, and that scan images at read positions using liner image pickup devices and thereby read the entire image, there are two main types of schemes. Note that generally a direction in which the one-dimensional image pickup devices are arranged is called a main scanning direction and a direction in which scanning is performed is called a sub-scanning direction.
One of the two types of schemes is a scheme in which the entire image in the main scanning direction is reduced in size and transferred onto the image pickup devices by a monocular lens, and is mainly used in copiers to read a front side. In this scheme, normally, the entire document is scanned such that the image pickup devices and the lens located on the document side are fixed and only mirrors are caused to move in the sub-scanning direction. This scheme has an advantage in that since a depth of focus (called a depth of field) on the document side is as large as the order of several mm, e.g., 6 mm, a document can be read even when the document is not in contact with a document reading surface of the copier. For example, even in the case where a document with a seam of a book or the like cannot be brought into contact with the document reading surface, there is an advantage of being able to read the document without going out of focus. Thus, this scheme is mainly used to read the front side of the document in copiers. There are various Patent Citations derived from this scheme but a Patent Citation 1, for example, is given as an example (hereinafter referred to as conventional method 1).
The other one of the two types of schemes is a scheme in which an image in the main scanning direction is divided into a plurality of images and the images are read by compound lenses, and is normally called a contact-type image pickup device. This scheme is used for reading a back side of the document in copiers, reading of documents in facsimiles, a bill recognition sensor, a scanner for a personal computer, and the like, and is characterized to be small in size. A conventional technique that is currently the mainstream of optical systems of the contact-type image pickup device is disclosed, for example, in a Patent Citation 2. The Patent Citation 2 discloses an image-scanning device that obtains an equal magnification erected image using, as compound lenses (referred to as a rod lens array in the Document), an array of a plurality of rod lenses, each of which has a refractive index profile in a radius direction thereof defined by a certain function (hereinafter referred to as conventional method 2).
As another example of representative schemes for optical systems of the contact-type image pickup device, there is one disclosed, for example, in a Patent Citation 3. In this scheme, by lenses mounted for respective cells divided in the main scanning direction, images in regions of the respective cells are reduced in size and transferred and then formed on image pickup devices. By combining output signals from the image pickup devices mounted for the respective cells, into images, images on a document surface are restored (hereinafter referred to as conventional method 3).
In addition, although not included in the above-described two representative schemes, a configuration using a reflecting mirror is disclosed in a Patent Citation 4. A document scanning device described in the Patent Citation 4 belongs to a scheme in which, instead of transferring images of a document surface by a lens array and reading them by image pickup devices, images on a document are directly read by image pickup devices mounted very close to the document. In the document scanning device, light reflected from the document surface is caused to propagate within a thin transparent substrate and is collected to the image pickup devices by a concave mirror. As is also clear from FIG. 2 in the Patent Citation 4, the concave mirror has a cylindrical shape and thus does not have an image-forming action in the main scanning direction (hereinafter referred to as conventional method 4).    Patent Citation 1: Japanese Unexamined Patent Publication No. 10-308852    Patent Citation 2: Japanese Unexamined Patent Publication No. 8-204899    Patent Citation 3: Japanese Unexamined Patent Publication No. 5-14600    Patent Citation 4: Japanese Unexamined Patent Publication No. 3-99576 (P2, the third paragraph of the upper-right column)